view mercurial/cext/mpatch.c @ 45059:79f6f9fa18c1

documentation: add some internals documentation about bid merge

This is an important algorithm that was only documented on the wiki so far.

Some update to the algorithm (and associated doc) is to expected in the future
since the bid merge algorithm is bug-ridden when it comes to file deletion comes
to play.

Differential Revision: https://phab.mercurial-scm.org/D8711

/*
 mpatch.c - efficient binary patching for Mercurial

 This implements a patch algorithm that's O(m + nlog n) where m is the
 size of the output and n is the number of patches.

 Given a list of binary patches, it unpacks each into a hunk list,
 then combines the hunk lists with a treewise recursion to form a
 single hunk list. This hunk list is then applied to the original
 text.

 The text (or binary) fragments are copied directly from their source
 Python objects into a preallocated output string to avoid the
 allocation of intermediate Python objects. Working memory is about 2x
 the total number of hunks.

 Copyright 2005, 2006 Matt Mackall <[e-mail]>

 This software may be used and distributed according to the terms
 of the GNU General Public License, incorporated herein by reference.
*/

#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include <stdlib.h>
#include <string.h>

#include "bitmanipulation.h"
#include "compat.h"
#include "mpatch.h"
#include "util.h"

static char mpatch_doc[] = "Efficient binary patching.";
static PyObject *mpatch_Error;

static void setpyerr(int r)
{
	switch (r) {
	case MPATCH_ERR_NO_MEM:
		PyErr_NoMemory();
		break;
	case MPATCH_ERR_CANNOT_BE_DECODED:
		PyErr_SetString(mpatch_Error, "patch cannot be decoded");
		break;
	case MPATCH_ERR_INVALID_PATCH:
		PyErr_SetString(mpatch_Error, "invalid patch");
		break;
	}
}

struct mpatch_flist *cpygetitem(void *bins, ssize_t pos)
{
	Py_buffer buffer;
	struct mpatch_flist *res = NULL;
	int r;

	PyObject *tmp = PyList_GetItem((PyObject *)bins, pos);
	if (!tmp) {
		return NULL;
	}
	if (PyObject_GetBuffer(tmp, &buffer, PyBUF_CONTIG_RO)) {
		return NULL;
	}
	if ((r = mpatch_decode(buffer.buf, buffer.len, &res)) < 0) {
		if (!PyErr_Occurred()) {
			setpyerr(r);
		}
		res = NULL;
	}

	PyBuffer_Release(&buffer);
	return res;
}

static PyObject *patches(PyObject *self, PyObject *args)
{
	PyObject *text, *bins, *result;
	struct mpatch_flist *patch;
	Py_buffer buffer;
	int r = 0;
	char *out;
	Py_ssize_t len, outlen;

	if (!PyArg_ParseTuple(args, "OO:mpatch", &text, &bins)) {
		return NULL;
	}

	len = PyList_Size(bins);
	if (!len) {
		/* nothing to do */
		Py_INCREF(text);
		return text;
	}

	if (PyObject_GetBuffer(text, &buffer, PyBUF_CONTIG_RO)) {
		return NULL;
	}

	patch = mpatch_fold(bins, cpygetitem, 0, len);
	if (!patch) { /* error already set or memory error */
		if (!PyErr_Occurred()) {
			PyErr_NoMemory();
		}
		result = NULL;
		goto cleanup;
	}

	outlen = mpatch_calcsize(buffer.len, patch);
	if (outlen < 0) {
		r = (int)outlen;
		result = NULL;
		goto cleanup;
	}
	result = PyBytes_FromStringAndSize(NULL, outlen);
	if (!result) {
		result = NULL;
		goto cleanup;
	}
	out = PyBytes_AsString(result);
	/* clang-format off */
	{
		Py_BEGIN_ALLOW_THREADS
		r = mpatch_apply(out, buffer.buf, buffer.len, patch);
		Py_END_ALLOW_THREADS
	}
	/* clang-format on */
	if (r < 0) {
		Py_DECREF(result);
		result = NULL;
	}
cleanup:
	mpatch_lfree(patch);
	PyBuffer_Release(&buffer);
	if (!result && !PyErr_Occurred()) {
		setpyerr(r);
	}
	return result;
}

/* calculate size of a patched file directly */
static PyObject *patchedsize(PyObject *self, PyObject *args)
{
	long orig, start, end, len, outlen = 0, last = 0, pos = 0;
	Py_ssize_t patchlen;
	char *bin;

	if (!PyArg_ParseTuple(args, PY23("ls#", "ly#"), &orig, &bin,
	                      &patchlen)) {
		return NULL;
	}

	while (pos >= 0 && pos < patchlen) {
		start = getbe32(bin + pos);
		end = getbe32(bin + pos + 4);
		len = getbe32(bin + pos + 8);
		if (start > end) {
			break; /* sanity check */
		}
		pos += 12 + len;
		outlen += start - last;
		last = end;
		outlen += len;
	}

	if (pos != patchlen) {
		if (!PyErr_Occurred()) {
			PyErr_SetString(mpatch_Error,
			                "patch cannot be decoded");
		}
		return NULL;
	}

	outlen += orig - last;
	return Py_BuildValue("l", outlen);
}

static PyMethodDef methods[] = {
    {"patches", patches, METH_VARARGS, "apply a series of patches\n"},
    {"patchedsize", patchedsize, METH_VARARGS, "calculed patched size\n"},
    {NULL, NULL},
};

static const int version = 1;

#ifdef IS_PY3K
static struct PyModuleDef mpatch_module = {
    PyModuleDef_HEAD_INIT, "mpatch", mpatch_doc, -1, methods,
};

PyMODINIT_FUNC PyInit_mpatch(void)
{
	PyObject *m;

	m = PyModule_Create(&mpatch_module);
	if (m == NULL)
		return NULL;

	mpatch_Error =
	    PyErr_NewException("mercurial.cext.mpatch.mpatchError", NULL, NULL);
	Py_INCREF(mpatch_Error);
	PyModule_AddObject(m, "mpatchError", mpatch_Error);
	PyModule_AddIntConstant(m, "version", version);

	return m;
}
#else
PyMODINIT_FUNC initmpatch(void)
{
	PyObject *m;
	m = Py_InitModule3("mpatch", methods, mpatch_doc);
	mpatch_Error =
	    PyErr_NewException("mercurial.cext.mpatch.mpatchError", NULL, NULL);
	PyModule_AddIntConstant(m, "version", version);
}
#endif